Hello,
Thank you for the update.
No problem. Once the website is moved to public hosting, please provide us with the WP-Admin access details in the Private Content field, and we will check the Checkout page and compare its layout settings with the rest of the website.
In the meantime, you can also check Theme Settings > General > Layouts and the individual Checkout page settings to make sure no boxed/container option has been applied specifically to the Checkout page.
Once we have access to the live site, we can investigate this more precisely and determine what is causing the header, footer, and checkout content to use a constrained width only on checkout.
Best Regards